The present invention relates to a cooling system for a fuel cell, and more particularly, to a cooling system equipped with an ion exchanger for removing ions from coolant.
Recently, the polymer electrolyte membrane fuel cell has drawn attention as a power source of an electric vehicle etc., because of its cleanness and excellent energy efficiency. Such a fuel cell is a kind of generator which electrochemically generates electricity with the supply of fuel gas (hydrogen) and oxidant gas (air). The temperature of the fuel cell increases when the fuel cell generates electricity.
Polymer electrolyte membrane fuel cell has an optimum operating temperature range, and a predetermined temperature of coolant (approximately 70xc2x0 C.) is supplied to operate the fuel cell within the optimum temperature range (approximately 85xc2x0 C.).
Meanwhile, when ion exchange resin is thermally decomposed, substance yielded in the thermal decomposition deteriorates conductivity. The control temperature of the fuel cell, i.e. the control temperature of coolant to be supplied to the fuel cell, is higher (e.g., 70xc2x12xc2x0 C.) than the temperature (heat resistance temperature of the ion exchange resin) at which the ion exchange resin can be operated for a long period of time under favorable conditions. Supplying high temperature coolant to the ion exchanger will result in a deterioration of the ion exchange resin. Therefore, low temperature coolant is supplied to the ion exchanger. The ion exchanger also requires more than a predetermined amount of coolant to be continuously flowed through the ion exchanger for the purpose of controlling the conductivity.
A conventional cooling system 50 for a fuel cell is shown in FIG. 4. The cooling system 50 includes a first cooling passage (cooling passage) 51 for circulating and feeding first coolant (coolant) to the fuel cell FC. Provided along the first cooling passage 51 are a first circulating pump 53, a heat exchanger 58, a bypass line 51A bypassing the heat exchanger 58, and a heat regulator (thermostat valve) 54 by which cold coolant (approximately 60xc2x0 C.) flowing through the heat exchanger 58 and warm coolant (approximately 80xc2x0 C.) flowing through the bypass line 51A are mixed so that the temperature of a first coolant is adjusted. By this arrangement of the cooling system 50, the first coolant (coolant) that is adjusted at the temperature of 70xc2x12xc2x0 C. by the heat regulator 54 can be supplied to the fuel cell FC.
In order to keep the conductivity of the first coolant lower, the cooling system 50 is arranged such that a part of the cold first coolant flowing through the heat exchanger 58 flows into the ion exchanger 55 to remove ions and then returns to the intake side of the first circulating pump 53. In this figure, reference numeral 51B denotes a supply line for supplying the ion exchanger 55 with the first coolant that has flowed through the heat exchanger 58, and reference numeral 51C denotes a return line for returning the first coolant to the first cooling passage 51 after the ion exchanger 55 removes ions from the first coolant. Further, reference numeral 52 denotes a second cooling passage 52 for cooling the first cooling passage 51, reference numeral 56 denotes a secondary circulating pump, and reference numeral 57 denotes a radiator for cooling the second cooling passage 52.
In this arrangement of the cooling system 50, it is possible to constantly supply the ion exchanger 55 with cold first coolant that has flowed through the heat exchanger 58, when compared with the arrangement where first coolant is supplied to the ion exchanger 55 from the downstream of the heat regulator 54 after warm first coolant and cold first coolant are mixed together and the temperature of the mixed first coolant is adjusted.
In the conventional cooling system 50 where a part of the first coolant (coolant) is separated from the upstream of the heat regulator 54, if the temperature of the first cooling passage 51 increases entirely due to an increased load of the fuel cell FC (and/or deteriorated performance of the heat exchanger 58), cold first coolant to be supplied to the heat regulator 54 becomes insufficient. Therefore, the heat regulator 54 performs temperature adjustment beyond its threshold limit, which makes it impossible for the cooling system 50 to supply the fuel cell FC with first coolant that has been adjusted to a proper temperature. Especially, when the fuel cell FC is used under conditions of rapid load changes, such as in the case where the fuel cell FC is mounted on an electric vehicle, such a problem would occur frequently. It may be possible to return first coolant that has flowed through the ion exchanger 55 to the downstream of the heat regulator 54. However, in terms of controlling temperature, it is not preferable that first coolant whose temperature has been adjusted by the heat regulator 54 is mixed with first coolant whose temperature has not been adjusted.
In view of the above, the primary object of the present invention is to provide a cooling system for a fuel cell, which can smoothly adjust the temperature of coolant, and which can supply an ion exchanger with a sufficient amount of coolant at temperatures as low as possible, to thereby drive the fuel cell under favorable conditions.
According to a first aspect of the present invention, a cooling system for a fuel cell includes: a heat exchanger which cools coolant discharged from the fuel cell; a heat regulator which adjusts a temperature of coolant to be supplied to the fuel cell after mixing coolant that has been cooled by the heat exchanger and coolant that has bypassed the heat exchanger together; an ion exchanger which removes ions from coolant with the use of ion exchange resin; and a supply control means which controls coolant to be supplied to the ion exchanger. The supply control means supplies the ion exchanger with coolant from a downstream of the heat regulator when the heat regulator operates beyond a coolant temperature adjustable range, and the supply control means supplies the ion exchanger with coolant that has been cooled by the heat exchanger from an upstream of the heat regulator when the heat regulator operates within the coolant temperature adjustable range.
In this construction of the cooling system, when the heat regulator operates within the coolant temperature adjustable range (controllable range), the supply control means supplies the ion exchanger with coolant (flowing through the heat exchanger) from the upstream of the heat regulator, so as to prevent the ion exchanger from being deteriorated by heat. When the heat regulator operates beyond the coolant temperature adjustable range (controllable range), the supply control means supplies the ion exchanger with coolant from the downstream of the heat regulator, so that the temperature adjustment can be performed reliably at the heat regulator without deficiency of the supply of coolant flowing through the heat exchanger. Because temperature adjustment has already been applied to coolant flowing downstream of the heat regulator, a part of the coolant can be taken out from the downstream of the heat regulator without affecting the heat regulator. To be more specific, cold coolant is supplied to the ion exchanger from the upstream of the heat regulator when the heat regulator operates within the temperature adjustable range, and a supply of cold coolant from the upstream of the heat regulator is stopped when the heat regulator operates beyond the temperature adjustable range. Therefore, temperature adjustment of the coolant can be performed in a reliable manner, and coolant at a temperature as low (cold) as possible can be supplied to the ion exchanger.
The supply control means described in the preferred embodiment includes two solenoid valves and the like, and when one of the two solenoid valves allows a flow of coolant, the other solenoid valve shuts off a flow of coolant.
The aforementioned cooling system may have a temperature detector which detects the temperature of coolant to be supplied to the fuel cell. Further, the supply control means supplies the ion exchanger with coolant that has bypassed the heat exchanger when the temperature of the coolant is equal to or lower than a predetermined temperature, and the supply control means supplies the ion exchanger with coolant that has been cooled by the heat exchanger when the temperature of the coolant is higher than the predetermined temperature.
In this construction of the cooling system, by detecting the temperature of coolant to be supplied to the fuel cell, it is possible to adjust the temperature of the coolant in a more reliable manner.